The present invention relates generally to sensors for media processing systems. In particular, the present invention is a capacitive sensing system for monitoring the locations of photographic film within an automatic film processor.
Automatic photographic film processing systems include a number of tanks which contain baths of liquid developing chemicals. Transport mechanisms which include driven rollers and guides are removably fitted within the tanks. Sheets of film are developed as they are driven through the tanks by the transport mechanisms. The sheets of film are guided between the transport mechanisms of adjacent tanks by crossover guides.
Sheets of film will occasionally get jammed within the transport mechanisms, or slide out of the transport mechanisms and into the tanks of developing chemicals. The operator of the film processor is alerted to these jammed and loose film occurrences by a film transport sensing system. Sensing systems of this type typically include sensors at the film input and output points of the processor. The sensors are coupled to a control system which is programmed with information characterizing the time it is expected to take the transport mechanisms to drive sheets of film between the input and output points. Jammed or loose film conditions are identified when the elapsed time between the receipt of detection signals from the input and output sensors exceeds the expected transport time. The control system stops the transport mechanisms and activates an alarm to alert the operator when these conditions are detected.
Since the processing tanks are relatively deep, the location of the jammed or loose film is often not immediately evident to the operator. A number of transport mechanisms must then be removed or disassembled to locate and correct the problem. This is an inconvenient task since the developing chemicals are corrosive and dangerous. This task can also result in damage to non-affected films within the processor.
The hostile environment within the processor also places constraints on the types of sensors which can be incorporated into the sensing system. Since the wet emulsion on the sheets of film is very soft, it can be scratched by physical contact sensors. Photoelectric detectors can cause undesired light fogging of the film.
It is evident that there is a continuing need for improved film position sensing systems and sensors used with the systems. The sensors must be relatively inexpensive, yet capable of accurately and non-destructively identifying the presence of film. A system which could pinpoint the location of jammed or loose film conditions within the processor would be especially desirable. Any such system and sensors must also be capable of withstanding the hostile chemical environment within the film processor.